Because of, for example, increased government interest in pollutants and air quality in, for example, urban areas, emissions standards and regulations have been formulated in many jurisdictions.
Such emissions standards often include sets of requirements that define acceptable limits for exhaust gas emissions from vehicles equipped with combustion engines. For example, emission levels for nitrogen oxides (NOx), carbohydrates (HC), carbon monoxide (CO) and particles are often regulated for most types of vehicles in such standards.
Undesired emissions can, for example, be reduced by reducing fuel consumption and/or via post-treatment (purification) of the exhaust gases produced by combustion in the combustion engine.
Exhaust gases from a combustion engine can, for example, be post-treated through the use of a so-called catalytic purification process. Different types of catalytic converters exist, and different types may be required for various fuels and/or to purify different types of exhaust gas components, and, in the case of at least nitrogen oxides NOx (such as NO and nitrogen dioxide, NO2), heavy vehicles often contain a catalytic converter in which an additive is supplied to the exhaust gas stream from the combustion in the combustion engine in order to reduce nitrogen oxides NOx (into mainly nitrogen gas and water vapor).
One commonly occurring type of catalytic converter to which additives are supplied consists of SCR (Selective Catalyst Reduction) catalytic converters. SCR catalytic converters use ammonia (NH3) or a compound from which ammonia can be generated/formed as an additive to reduce the level of nitrogen oxides NOx. The additive is injected into the exhaust gas stream resulting from the combustion engine, upstream of the catalytic converter.
The additive supplied to the catalytic converter is absorbed (stored) in the catalytic converter, whereupon nitrogen oxides NOx in the exhaust gases react with the ammonia stored in the catalytic converter. When supplying an additive, it is important that the amount of additive not be too great or too small. It is thus desirable for the supplied amount of additive to correspond to an anticipated amount of additive.